Christmas Is Coming

Western Australian Christmas tree, Nuytsia floribunda, in flower. Photo from Esperance Blog.

In the last few weeks, the Christmas trees near our house have begun flowering. Nuytsia floribunda, the Western Australian Christmas tree, is without doubt one of the most remarkable plants found in the Perth region. Even coming into my fourth summer here, the sight of a Christmas tree still never fails to catch my attention. Why are they so remarkable?

First, there's the appearance of the tree itself. For most of the year, a Christmas tree is a fairly insignificant, often decidedly scraggly, dark green tree. It can reach a height of about ten metres, but I don't think most of the ones I've seen (and they're not uncommon in remnant bush patches) have been anywhere near so tall. You could be quite readily forgiven for overlooking them. But all that changes about the beginning of November, when they begin to flower - heavily. What was a point of scraggly green becomes a blazing firebrand of burnished gold, as the entire tree becomes covered in individually tiny, but collectively magnificent, yellow flowers. The flowers remain during the next few months, past the end of December (hence, of course, the name), blazing like a beacon all the while.

As noteworthy as this blazing cheer alone would be, Nuytsia has even more points worthy of fascination to draw the attention. Despite its attraction, Nuytsia is rarely grown as a garden plant, and most attempts to do so meet with little success. Why is Nuytsia so recalcitrant? Because this showy shrub is something of a floral femme fatale, with dark secrets hidden beneath the soil. Nuytsia is a parasite, with a double Christmas connection - it is the world's largest mistletoe.


Plant roots with attached white Nuytsia haustoria. Photo from here (which also has a photo of Nuytsia haustoria attached to roots of broomrape, Orobanche minor, itself a holoparasite).

Mistletoes of the family Loranthaceae belong to a clade called Santalales that also includes such plants as sandalwoods. Most Santalales, including mistletoes, are hemiparasites - that is, they derive at least some of their nutrient requirements from other plants, but still retain chlorophyll and produce some of their nutrients themselves. The Santalales also include some non-parasitic species that form the paraphyletic outgroup to the parasitic clade (Nickrent & Malécot, 2001), and recent studies suggest that the holoparasitic (entirely parasitic) Balanophoraceae may also belong to the Santalales (Nickrent et al., 2005). As it is, the parasitic Santalales are, by any measure, the most successful clade of plant-parasitic angiosperms in existence.

The majority of mistletoes are aerial parasites, growing directly on the trunk or branches of the host tree. Nuytsia, however, is a root parasite. It grows in the ground like a normal tree, but its roots hunt through the soil in search of the roots of other plants to latch onto and parasitise. Once the roots come into contact with a potential host, they start growing a pair of lateral projections that wrap around the host root, forming a doughnut-shaped haustorium (nutrient-absorptive tissue). On the inside of the haustorium, a sharp, hard structure develops shaped like a pair of horns, or the blades of a pair of scissors (Calladine & Pate, 2000). The sharp inside edges of this structure quickly cut through the host root (exactly like a pair of scissors), severing it into two parts, and the haustorium then grows over the exposed ends of the roots, diverting the flow of nutrients and water away from the host and into the waiting Christmas tree. Nuytsia does not seem to be choosy when it comes to hosts - it may parasitise any trees within a radius of up to 150 m, but it may also parasitise smaller plants, even grass (allowing it to survive in locations without other trees). Nuytsia have also been recorded attempting to grow haustoria around buried twigs, small stones, and even electrical cables!


Cross-section of a Nuytsia haustorium, showing the hardened structure used to cut through the host root. Photo by Stephan Imhof.

Phylogenetic analysis shows that root parasitism represents the basal condition for parasitic Santalales, with multiple origins of aerial parasitism within the clade (Vidal-Russell & Nickrent, 2008). In fact, the root-parasitic Nuytsia, as well as being the largest member of the family, is also the sister taxon to all other Loranthaceae, making it a fascinating taxon phylogenetically as well as ornamentally and ecologically. So the next time any of you see a Christmas tree in flower, stop for a moment and consider how there's a lot more to it than you can see, hidden below the surface.

REFERENCES

Calladine, A., & J. S. Pate. 2000. Haustorial structure and functioning of the root hemiparastic tree Nuytsia floribunda (Labill.) R.Br. and water relationships with its hosts. Annals of Botany 85: 723-731.

Nickrent, D. L., J. P. Der & F. E. Anderson. 2005. Discovery of the photosynthetic relatives of the "Maltese mushroom" Cynomorium. BMC Evolutionary Biology 5: 38 (http://www.biomedcentral.com/1471-2148/5/38).

Nickrent, D. L., & V. Malécot. 2001. A molecular phylogeny of Santalales. In Proceedings of the 7th International Parasitic Weed Symposium (A. Fer, P. Thalouarn, D. M. Joel, L. J. Musselman, C. Parker, and J. A. C. Verkleij, eds.) pp. 69-74. Faculté des Sciences, Université de Nantes, Nantes, France.

Vidal-Russell, R., & D. L. Nickrent. 2008. The first mistletoes: origins of aerial parasitism in Santalales. Molecular Phylogenetics and Evolution 47 (2): 523-537.